In dissecting some of the F₂ birds from this cross I noticed that the testis of the male was often more flattened than is the testis of the typical male bird, that it was often somewhat pear-shaped, and that frequently it was in part or entirely black. Recalling that male Sebrights are said to be often partially sterile, the idea naturally suggested itself that these birds are hen-feathered because the testes have assumed some of the characteristics of the ovary. It had long been supposed, and had been finally established by Goodale, that the presence of the ovary in the female suppresses her potential development of plumage, for when the ovaries of the hen are diseased or removed she develops the plumage of the male. This reasoning led me to try the experiment of castrating the hen-feathered males in order to see if they would become cock-feathered. The outcome was immediately apparent; the new feathers were those of the cock bird. While the “reasoning” that led to the experiment is open to serious question, nevertheless the “hint” furnished by the unusual condition of the testis led finally to the discovery that luteal cells were present in abundance in the testes of the male Sebright like those present only in the females of other breeds. Whether or not the shape of the testis of the Sebright, that is sometimes like that of the ovary, is connected with the unusual abundance of luteal cells in the testis I do not know. If so, then the hint that came from their shape was not so unreasonable as appears at first sight. The birds first operated upon were adult F₁ and F₂ hen-feathered birds. The first one done by myself died, but a few, whose testes were removed by Dr. H. D. Goodale at my request, lived and changed to cock-feathered birds. Since then I have operated successfully on a number of F₁ and F₂ birds, as well as Sebright males. In these operations I have had throughout the assistance of Dr. A. H. Sturtevant and for two years the assistance of Dr. J. W. Gowen also. I wish to express my appreciation of their help and advice, for without it I doubt whether I could have carried out the work successfully. Since the main interest attaches to the Sebright experiments, they will be described first, although they were the last to be performed. CASTRATION OF SEBRIGHTS.